1. Field of the Invention
This invention relates to a stereoscopic microscope for use as a microscope for medical operations, and in particular to a stereoscopic microscope having the observation optic axis thereof deflected in the direction of the observer's line of vision.
2. Related Background Art
A conventional stereoscopic microscope having the observation optic axis thereof deflected by about 45.degree. so as to form an angle of depression to facilitate observation will hereinafter be described on the basis of an example of the prior art shown in FIGS. 1 and 2 of the accompanying drawings. This example of the prior art is designed such that a portion E to be examined is stereoscopically observed by the observer O through relay lens systems such as zoom magnification changing optical systems 2L, 2R, overlooking prisms 3L, 3R for deflecting the optic axes by 45.degree. for the formation of an angle of depression and having an image erecting function, eye width adjusting prisms 4L, 4R parallel-moving the optic axes and effecting adjustment of the eye width and eyepieces 5L, 5R (2R, 3R, 4R and 5R not shown) disposed in succession on a pair of parallel left and right optic axes rearward of a common objective 1.
Where an additional instrument such as a view measuring mirror, a camera or a TV camera is mounted on this stereoscopic microscope, beam splitters 6L, 6R (6R not shown) are inserted in the intermediate portion of the observation optical path, that is, between the magnification changing optical systems 2L, 2R and the overlooking prisms 3L, 3R. Thus part of the observation light beam is deflected in a leftward direction substantially orthogonal to the optic axes by the light beam distributing surface 6a thereof. In some cases, these beam splitters 6L, 6R may be disposed in only one of the left and right optical systems.
FIG. 1B is an optical path illustration showing the manner of reflection of the light beam in the overlooking prism 3L. This overlooking prism 3L has effective surfaces 3a, 3b, 3c and 3d, of which the surfaces 3c and 3d provide a 90.degree. roof prism. Accordingly, the light beam from the magnification changing optical system 2L below the overlooking prism 3L enters the lower surface 3a of the overlooking prism 3L and is totally reflected by the surface 3b, whereafter it is totally reflected by the surface 3c, travels toward the surface 3d and is totally reflected by the surface 3a, whereafter it emerges from the surface 3b and is deflected by 45.degree. in a vertical direction with respect to the incident optic axis to the surface 3a. FIG. 1C is a plan view of the overlooking prisms 3L and 3R. Light beams having entered these overlooking prisms from positions corresponding to the magnification changing optical systems 2L and 2R indicated by broken lines are deflected by 45.degree. in a vertical direction by the overlooking prisms 3L and 3R, and enter the eye width adjusting prisms 4L and 4R disposed rearwardly thereof as left and right light beams L and R parallel to each other.
FIG. 2 is a side view of a twice-reflection overlooking prism 3' used instead of a four-times reflection Schmidt prism 3. A light beam having entered the surface 3e of this overlooking prism 3' is totally reflected by the surface 3f thereof and is further totally reflected by the surface 3e and emerges from the surface 3g thereof, whereby it is deflected by 45.degree.. However, this twice-reflection overlooking prism 3' is not a roof prism and has image erecting function. Therefore either the eye width adjusting prism 4 disposed rearwardly thereof must be a prism having an erecting function, such as a poloprism, or an optical system having an erecting function must be inserted into the optical path forward of the overlooking prism 3'.
Thus, in the stereoscopic microscope according to the prior art, light beams entering the eyes of the observer O are parallel to each other and this forces the observer O to assume a state different from the normal naked eye observation state. That is, in the naked eye observation state, the left and right eyes of the observer O have an angle of convergence to see a point, whereas when observation through the microscope is to be effected, convergence must be effected as if said point was at infinity. This makes it difficult for the observer O to effect stereoscopic observation and at the same time, excessively fatigues the observer during observation.
A stereoscopic microscope in which an overlooking prism is rotatively displaced in a plane perpendicular to the optic axis so as to provide a predetermined angle of convergence is proposed in U.S. application Ser. No. 788,785.
However, in these conventional apparatuses, light beam distributing means such as the beam splitters 6L and 6R in FIG. 1 are provided in the observation optical path discretely from the overlooking prisms 3L and 3R so as to form an additional optical path for mounting therein an additional instrument such as a TV camera or other recording apparatus for recording the state of an operation or the like, and this leads to the bulkiness of the apparatus and poor operability of the apparatus. Further, usually, the light beam distributing means are disposed in the observation optical path between the magnification changing optical systems 2L, 2R and the overlooking prisms 3L, 3R and therefore, the apparatus is subject to mechanical limitations, and the direction of deflection of the distributed light beam unavoidably becomes fixed, and this limits the degree of freedom of mounting of the additional instrument.
If overlooking prisms having no image erecting function are used as the overlooking prisms, the image erecting function will have to be performed by other optical member, and for the simplification of the construction of the apparatus, prisms having the image erecting function are desired.